1. Field of the Invention
This invention relates generally to apparatus and methods for analyzing the chemical composition of formation fluids, and more particularly, to apparatus and method for using infrared spectral analysis to determine the quantities of methane, water and various types of oils in a formation fluid.
2. Description of the Related Art
In the oil and gas industry, formation testing tools have been used for monitoring formation pressures along a wellbore, obtaining formation fluid samples from the wellbore and predicting performance of reservoirs around the wellbore. Such formation testing tools typically contain an elongated body having an elastomeric packer that is sealingly urged against the zone of interest in the wellbore to collect formation fluid samples in storage chambers placed in the tool.
During drilling of a wellbore, a drilling fluid (xe2x80x9cmudxe2x80x9d) is used to facilitate the drilling process and to maintain a pressure in the wellbore greater than the fluid pressure in the formations surrounding the wellbore. This is particularly important when drilling into formations where the pressure is abnormally high: if the fluid pressure in the borehole drops below the formation pressure, there is a risk of blowout of the well. As a result of this pressure difference, the drilling fluid penetrates into or invades the formations for varying radial depths (referred to generally as invaded zones) depending upon the types of formation and drilling fluid used. The formation testing tools retrieve formation fluids from the desired formations or zones of interest, test the retrieved fluids to ensure that the retrieved fluid is substantially free of mud filtrates, and collect such fluids in one or more chambers associated with the tool. The collected fluids are brought to the surface and analyzed to determine properties of such fluids and to determine the condition of the zones or formations from where such fluids have been collected.
One feature that all such testers have in common is a fluid sampling probe. This may consist of a durable rubber pad that is mechanically pressed against the rock formation adjacent the borehole, the pad being pressed hard enough to form a hydraulic seal. Through the pad is extended one end of a metal tube that also makes contact with the formation. This tube (xe2x80x9cprobexe2x80x9d) is connected to a sample chamber that, in turn, is connected to a pump that operates to lower the pressure at the attached probe. When the pressure in the probe is lowered below the pressure of the formation fluids, the formation fluids are drawn through the probe into the well bore to flush the invaded fluids prior to sampling. In some prior art devices, a fluid identification sensor determines when the fluid from the probe consists substantially of formation fluids; then a system of valves, tubes, sample chambers, and pumps makes it possible to recover one or more fluid samples that can be retrieved and analyzed when the sampling device is recovered from the borehole.
It is critical that only uncontaminated fluids are collected, in the same condition in which they exist in the formations. Commonly, the retrieved fluids are found to be contaminated by drilling fluids. This may happen as a result of a poor seal between the sampling pad and the borehole wall, allowing borehole fluid to seep into the probe. The mudcake formed by the drilling fluids may allow some mud filtrate to continue to invade and seep around the pad. Even when there is an effective seal, borehole fluid ( or some components of the borehole fluid) may xe2x80x9cinvadexe2x80x9d the formation, particularly if it is a porous formation, and be drawn into the sampling probe along with connate formation fluids.
U.S. Pat. No. 4,994,671 issued to Safinya et al. discloses a device in which visible and near infrared (IR) analysis of the fluids is done in the borehole, without having to transport recovered samples of the fluid to the surface for chemical analysis. The IR part of the electromagnetic spectrum (0.8 to 25 xcexcm wavelength region, or equivalently wavenumbers of 12500 to 400 cmxe2x88x921) of a substance contains absorption features due to the molecular vibrations of the constituent molecules. The absorptions arise from both fundamentals (single quantum transitions occurring in the mid-infrared region from 2.5-25.0 xcexcm) and combination bands and overtones (multiple quanta transitions occurring in the mid- and the near-infrared region from 0.8-2.5 xcexcm). The position (frequency or wavelength) of these absorptions contain information as to the types of molecular structures that are present in the material, and the intensity of the absorptions contains information about the amounts of the molecular types that are present. To use the information in the spectra for the purpose of identifying and quantifying either components or properties requires that a calibration be performed to establish the relationship between the absorbances and the component or property that is to be estimated. For complex mixtures, where considerable overlap between the absorptions of individual constituents occurs, such calibrations must be accomplished using various chemometric data analysis methods.
In complex mixtures, each constituent generally gives rise to multiple absorption features corresponding to different vibrational motions. The intensities of these absorptions will all vary together in a linear fashion as the concentration of the constituent varies. Such features are said to have intensities which are correlated in the frequency (or wavelength) domain. This correlation allows these absorptions to be mathematically distinguished from random spectral measurement noise which shows no such correlation. The linear algebra computations which separate the correlated absorbance signals from the spectral noise form the basis for techniques such as Principal Components Regression (PCR) and Partial Least Squares (PLS). As is well known, PCR is essentially the analytical mathematical procedure of Principal Components Analysis (PCA), followed by regression analysis.
PCR and PLS have been used to estimate elemental and chemical compositions and to a lesser extent physical or thermodynamic properties of solids and liquids based on their mid- or near-infrared spectra. These chemometric methods involve: [1] the collection of mid- or near-infrared spectra of a set of representative samples; [2] mathematical treatment of the spectral data to extract the Principal Components or latent variables (e.g. the correlated absorbance signals described above); and [3] regression of these spectral variables against composition and/or property data to build a multivariate model. The analysis of new samples then involves the collection of their spectra, the decomposition of the spectra in terms of the spectral variables, and the application of the regression equation to calculate the composition/properties.
In Safinya et al. light the visible and near IR region is passed through the fluid sample. A spectrometer measures the spectrum of the transmitted and the back scattered light, and knowing the spectrum of the incident light, transmission and backscattered absorption spectra for the sample are determined. Using absorption spectra of water, gas, crude and refined oils, and drilling fluids, a least squares analysis is performed that models the observed spectra as a weighted sum of the spectra of its components, the least squares analysis giving the composition of the fluid in terms of weights of the various components.
Safinya et al. use only the visible and near IR regions that contain only harmonics and combinations of molecular vibrations. The harmonic and combination absorption bands are much weaker than the fundamental absorption bands, and, for this reason, transmission methods of sampling are used to detect absorption spectra. The path lengths through the sample that are necessary to get detectable measurements are large, being typically 5 mm. or more. Even at this length of transmission, signal levels are lower and the spectral analysis of the harmonics and combinations is complicate. Also of importance is the fact that in downhole applications, the presence of particulate matter, microscopic particles or bubbles leads to scattering. This scattering drastically increases the optical density and reduces the ability to detect spectral features of the sample. This effect is discussed below with reference to FIGS. 1 and 2.
As noted above, the fundamental absorbances corresponding to the functional groups of organic chemicals fall in the mid infrared region. The absorbances are generally strong and to use transmission methods on such fluids would require transmission paths of 25 xcexcm or less. With spacings of this magnitude it is difficult to get good fluid flow through the optical cell. In addition, there are noticeable interference fringes. The method and apparatus of this invention helps overcome this problem.
Diffuse and specular reflectance methods have been used in prior art applications of near infrared analysis. Diffuse reflectance measurements require a large solid angle of data collection with a relatively large illuminated area to average out sample inhomogeneities. Consequently, diffuse reflectance is not suitable for measurement of small samples at low flow rates that are characteristic of reservoir fluid monitoring.
Specular reflectance is used in U.S. Pat. No. 5,167,149 issued to Mullins et al. Disclosed therein is an invention for analyzing the composition of multiphasic formation fluids, and specifically for detecting the presence of gas in a flow stream that comprises oil, water, gas or particulates within the borehole. The apparatus comprises a flow line for containing the fluid and a light source for transmitting light towards the fluid in the flow line. A prism transmits light from the source to the fluid and forms an interface with the flow line. The interface reflects the light from the source and a detector array detects the light. The angle of incidence at which total reflection of the light takes place provides a measure of the refractive index of the fluid in contact with the prism surface. As the refractive index of gas and liquids are essentially different, the amount of gas in the fluid can be measured. Specular reflectance methods are effective in identifying the composition of a multiphasic fluid in terms of its constituent phases but are not particularly useful for identifying the chemical composition of a single phase. However, due to the fact that at sufficiently high pressures, the refractive index of the gas phase approaches the refractive index of liquid hydrocarbons, specular reflectance methods cannot be used to determine methane concentration at high pressures.
U.S. patent application Ser. No. 09/111,368 which is assigned to the owner of the present invention discloses the use of attenuated total reflectance (ATR) methods for analysis of the absorbances at fundamental frequencies for the analysis of borehole fluids, particularly methane, in a flow detector. ATR methods make use of the fact that when light is incident at an interface between a first medium of a higher refractive index and a second medium of lower refractive index, there exists a critical angle beyond which total reflectance of the light takes place within the first medium. However, even beyond the critical angle, an evanescent wave is propagated into the second medium with a characteristic depth of penetration dp into the second medium given by       d    p    =      λ          2      ⁢      π      ⁢              xe2x80x83            ⁢              n        21            ⁢                                                  sin              2                        ⁢            θ                    -                      n            21            2                              
where n 21=n2/n1, the ratio of the refractive indices of the second and first media and xcex is the wavelength of the radiation in vacuum and the angle xcex8 exceeds the critical angle.
The energy carried by this evanescent wave manifests itself as a decrease in the energy of the reflected wave, a measurable quantity. ATR methods thus effectively function like the equivalent of a transmission cell having a transmission length de given by de=0.5 (ds+dp) where       d    s    =                    n        21            ⁢      λ      ⁢              xe2x80x83            ⁢      cos      ⁢              xe2x80x83            ⁢      θ              π      ⁢              xe2x80x83            ⁢                        n          1                ⁡                  (                      1            -                          n              21              2                                )                    ⁢                                                  sin              2                        ⁢            θ                    -                      n            21            2                              
is the effective thickness for light polarized perpendicular to the plane of incidence and       d    p    =                    n        21            ⁢      λ      ⁢              xe2x80x83            ⁢      cos      ⁢              xe2x80x83            ⁢      θ              π      ⁢              xe2x80x83            ⁢                        n          1                ⁡                  (                      1            -                          n              21              2                                )                    ⁢                                                                  sin                2                            ⁢              θ                        -                          n              21              2                                      ⁡                  [                                                    (                                  1                  +                                      n                    21                    2                                                  )                            ⁢                              sin                2                            ⁢              θ                        -                          n              21              2                                ]                    
is the effective thickness for light polarized in the plane of incidence. Some calculations will show that the effective thickness is of the order of the wavelength of the light.
Unlike transmission techniques, ATR methods are relatively insensitive to the presence of small particles in the fluid. This advantage of ATR methods is brought out by comparing FIGS. 1 and 2 (Prior art). In both figures, the abscissa is the wavenumber and the ordinate is the absorption. In FIG. 1, the near infrared transmission spectrum of pure silicone oil 2 may be compared with that of silicone oil with 0.015% of TiO2 4 and of silicone oil with 0.125% of TiO2 6. As can be seen, the addition of even a small amount of an absorbing material like TiO2 makes greatly increases the absorption of light and makes it almost impossible to pick out the underlying xe2x80x9csignalxe2x80x9d. In comparison, using ATR techniques, as shown in FIG. 2, the mid infrared absorbance spectrum of pure silicone oil 12 is not significantly affected by the addition of TiO2 14 in quantities as large as 0.5%, much larger than the values used in FIG. 1.
All of the above methods also suffer from the drawback that the fluid sample being evaluated is in the borehole inside the measurement apparatus and the fluid sample recovered from the formation may be contaminated by borehole fluids even with the most careful of sampling techniques. Any analysis technique must therefore account for the presence of the drilling fluid and its absorbance spectrum. There is a need for an invention that accurately and speedily provides a measurement of the composition of formation fluids without contamination by drilling fluids and particulate matter. The present invention satisfies this need.
The present invention provides an apparatus and a method for determination of the composition of a formation fluid. An optical probe carrying a sapphire crystal is inserted into the formation so as to get past the mud cake, the flushed zone and the invaded zone and thus he in contact with virgin formation fluid. An acousto-optical tunable filter transmits a single wavelength of light from a broad band light source. This monochromatic light is carried by an optic fiber to the internal reflectance sapphire crystal where it undergoes total reflection at the crystal faces in contact with the formation fluid. An evanescent wave propagates into the fluid with a depth of penetration related to the absorption of the light in the fluid. The reflected light carries information about this absorption. A return fiber conveys the reflected light back to a spectrometer. A processor determines the absorption at the wavelength of the monochromatic light by comparing the energy in the reflected light to the energy in the incident light. By repeating this process at a number of different wavelengths, the absorption spectrum of the fluid is determined. Principal component analysis of this absorption spectrum using known absorption spectra of chemicals likely to occur in the fluid gives the composition of the fluid in terms of these chemicals.